Wheels driven by an electric motor are known. The electric motor can be provided in the car and drive the axis of the wheel to rotate the wheel. This requires the electric motor to be located in the car. Another possibility is to position the electric motor in the wheel itself. Such wheels having an in-wheel electric motor are known as well. Part of the electric motor is generally coupled to the rim of the wheel, the rim carrying the tire of the wheel. Another part of the in-wheel electric motor is generally positioned centrally in the wheel and will constitute a non-rotating part of the wheel. The non-rotating part of the electric motor and part of the electric motor coupled to the rim are generally referred to as stator and rotor, respectively. The interaction between rotor and stator is of magnetic nature, at least one of the rotor and stator comprising electromagnets.
Their mutual distance should be small enough for the electric motor to be able to deliver enough power and torque to the wheel, and preferably as small as practically possible. On the other hand, their mutual distance should be large enough to be able to absorb loads and mechanical impacts on the wheel. Further, large currents are required to provide the necessary power and torque to the motor. This causes a temperature rise of the electric motor and wheel, and cooling is required to limit the temperature rise. Having a maximum of torque available for a given electrical current is very advantageous. Various solutions have been proposed, but they are all quite complicated.